Papers by Author: Oliver Kirstein

Abstract: The ternary carbide Ti3SiC2 is the archetype of MAX phases. To date, MAX phases have proven difficult to synthesize as sufficiently large single crystals from which single crystal elastic constants might be obtained. Therefore, the elastic properties not only of Ti3SiC2 but other MAX phases are extensively studied by ab initio methods. Recently single crystal elastic constants were experimentally determined for the first time using neutron diffraction. The experiment revealed extreme shear stiffness which is not only quite rare in hexagonal materials but also strongly contradicts the predictions of all published MAX phase elastic constants from ab initio calculations. In the present paper we would like to show that such shear stiffness can possibly be supported by ab initio calculations and the calculated phonon dispersion along high symmetry directions.

Abstract: Forming of metal plates with a high-power laser beam is a flexible materials forming technique. Bending results from the establishment of a steep temperature gradient through the material thickness which leads to non-uniform thermal expansion/contraction and subsequently residual stresses. It is important to characterize these residual stresses as a function of process parameters such as line energy (LE) to optimize treatment conditions and to gain an insight into the mechanism of the formation of the final geometries. Non-destructive neutron diffraction measurements were carried out on ENGIN-X at ISIS and KOWARI at ANSTO to map the residual stress distribution around the heat affected zone (HAZ) of laser deformed mild steel plates for single and multiple passes as a function of line energy (LE), the primary laser forming process parameter. It was found that in the centre of the HAZ, longitudinal residual stresses are tensile and dominant, transverse stresses are predominantly tensile and normal stresses are compressive and close to zero. The residual strain in longitudinal direction increased with LE and number of passes until yielding. Even higher heat input decreased the magnitude of the cusp, but not its total height. The comparison of a stress-free reference sample, measured at both facilities, showed a small discrepancy in the lattice spacing corresponding to ~ 85 μstrain which is insignificant with respect to the experimental values measured.

Abstract: Kowari is a neutron diffractometer at ANSTO’s research reactor OPAL. The instrument is dedicated to investigate residual strains and stresses in engineering samples or new materials. Besides its usage in engineering/strain-scanning it is more and more frequently being used for obtaining texture information related to materials science applications. In particular the possibility of using the gauge volume defining slit systems allows extracting local information from the diffraction peak, i.e. position, intensity, and width, without the need to cut the sample specimen. The instrument obtained its operating license in August 2008 to briefly describe the instruments parameters and capabilities, and give examples of two typical applications since it went from commissioning into user mode.

Abstract: In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. The determination of global texture and the local variation of texture for example by inhomogeneous deformation are very important due to the coherence between the texture and the physical and mechanical properties of materials.

Abstract: The ternary ceramic Ti3AlC2 has an interesting combination of electrical, thermal and mechanical properties. Single crystal elastic constants under the Reuss approximation for the micromechanical state were obtained by analysing the shifts of neutron diffraction peaks while a polycrystalline sample was subjected to a compressive load varying from 5 to 300 MPa. The values of Young’s modulus and Poisson’s ratio computed from the single crystal compliances are in good agreement with those obtained directly from strain gauges and from the average changes in the a and c unit cell parameters.

Abstract: The Australian Nuclear Science and Technology Organisation (ANSTO) has recently started commissioning the new Australian Research Reactor OPAL that has replaced the old HIFAR reactor in January 2007. At the first stage, the new reactor will provide neutrons to several neutron scattering instruments. Among them is the residual stress diffractometer Kowari that was designed to study engineering problems related to residual stresses as well as allow material science research using neutron diffraction. We give an update on the progress of the instrument’s installation and commissioning and present an example to illustrate how neutron diffraction can be used to obtain information about residual stresses in a flash butt welded plate.

Abstract: Neutron scattering using diffraction techniques is now recognised as the most precise and reliable method of mapping sub-surface residual stresses in materials and industrial devices. It was therefore decided to build the dedicated strain scanner KOWARI among the first suite of instruments for the new Australian Replacement Research Reactor OPAL. In order to support the new instrument and familiarize the Australian user community with the technique a „Neutrons for Engineering“ project has been established to provide a fully integrated residual stress service. To make use of the neutrons at Australias HIFAR reactor the existing three-axis spectrometer has been refurbished and modified to allow strain measurements. The Australian Strain Scanner (TASS) will operate until the new strain scanner becomes operational in 2006. Apart from giving an overview about the technique and properties of the new strain scanner we would like to present and discuss two case studies: a) residual-stresses obtained from a break-disc of a car and b) residual-stresses within a flash-butt welded plate cut out of a railway.

Abstract: The Australian Nuclear Science and Technology Organisation, ANSTO, (http:\\www.ansto.gov.au) has initiated a “Neutrons for Engineering” project to provide an integrated residual stress service to Australian industry and academia. The service is based around measurements of residual stress using neutrons on a newly-refurbished instrument on the HIFAR research reactor. In addition to the neutron measurements there is a range of expertise available on the ANSTO site to solve residual stress problems using other techniques including hole-drilling, strain-gauging, and x-ray diffraction, as well as capabilities for finite element modeling and mechanical testing. In this paper we describe briefly the existing and future facilities at ANSTO for neutron strain scanning and present some benchmark results for the HIFAR strain scanner.